Recent years have seen the increasing popularity of digital cameras among other camera devices, and the number of digital cameras produced is increasing yearly. In general, the digital cameras are roughly classified into single-lens reflex type and compact type. Recently, the digital cameras of the single-lens reflex type are increasingly being shipped, but as for the digital cameras of the compact type, small and slim digital cameras are also still popular. To achieve such size and thickness reduction of the cameras, Japanese Laid-Open Patent Publication No. 2002-350943 proposes technology for improving, for example, the structure of a battery chamber with battery connectors directly attached to a switch.
While the size and the thickness are being reduced, the number of functions tends to be increased, so that technology for internal circuits, etc., is becoming complicated and advanced. To meet the demands to reduce the device size and thickness, and increase the circuit scale under such circumstances, it is necessary to deal with such problems as more efficient packaging of internal parts, and reduction in number of man-hours spent for part integration. In particular, for small movable parts, such as a shutter button, it is necessary to achieve simple and reliable assembly work.
FIG. 11 illustrates the cross-sectional configuration of a conventional switch pressing mechanism intended for size and thickness reduction, in which a shutter button is biased by a coil spring, rather than by a leaf switch. The switch pressing mechanism RSp includes a shutter button 2, a guide member 4, a guide member 5, and a coil spring 6p. 
The guide member 4 is composed of a cylindrical portion, and a ring portion, which is provided in such a manner as to be substantially perpendicular to the cylindrical portion and extend toward the central axis of the switch pressing mechanism RSp. In addition, the guide member 4 is attached at the ring portion to an upper circumferential surface of an opening portion 31 provided in a portion of a housing 3 of a digital camera 1p. 
The shutter button 2 includes a disk-shaped portion 21, and an outer ring portion 22 extending around the disk-shaped portion 21. Note that the outer ring portion 22 has a step portion 23 provided at its outer circumference. The shutter button 2 is formed, such that the diameter of the outer circumference, including the step portion 23, is greater than the inner diameter of the ring portion of the guide member 4, and smaller than the inner diameter of the cylindrical portion. The shutter button 2 has a columnar boss 24 at the center of its back side, the columnar boss 24 being defined with an outer diameter D24 smaller than a diameter D51 of a hole 51 provided at the center of the guide member 5.
The guide member 5 is provided in the shape of a disk having a flat portion 52. The guide member 5 has the hole 51 provided at the center, the hole 51 being defined with the diameter D51 greater than the outer diameter D24 of the boss 24. The guide member 5 is attached at the upper surface of its outer circumference to a lower circumferential surface of the opening portion 31. Note that a switch 7, which is attached to a circuit board 8, is disposed below the guide member 5 so as to be opposed to the hole 51.
The coil spring 6p is defined with a diameter D6 smaller than the inner diameter of the shutter button 2, and greater than the diameter D51 of the hole 51. Note that the diameter D6 of the coil spring 6p is a mean diameter between the outer diameter and the inner diameter. Although not shown in the figure due to limitations of space, the outer and inner diameters of the coil spring 6p are denoted by D6o and D6i, respectively.
The shutter button 2 is fitted in the guide members 4 and 5, with the boss 24 being inserted into the inner circumferential portion of the coil spring 6p. Concretely, the shutter button 2 is engaged with the guide member 4, such that the outer circumference of the disk-shaped portion 21 is placed at a predetermined clearance from the inner circumference of the ring portion of the guide member 4. The step portion 23 is in contact with the bottom surface of the ring portion, so that the coil spring 6p is compressed between the bottom surface of the guide member 4 and the top surface of the guide member 5, biasing the shutter button 2 by a predetermined force toward the direction opposite to the direction indicated by arrow Y (hereinafter, referred to as the “direction Y”). That is, the shutter button 2 is held with the top surface of the step portion 23 being in contact with the bottom surface of the ring portion. As a result, the shutter button 2 is held so as not to slip off.
In the switch pressing mechanism RSp thus configured, the shutter button 2 is pressed in the direction Y by a human finger or suchlike, so that the boss 24 moves in the direction Y to touch the switch 7, while resisting the biasing force of the coil spring 6p. In general, a two-step click switch is used as the switch 7, and therefore by pressing the shutter button 2 consecutively, the state of the switch 7 sequentially changes to “half-pressed” and then to “completely-pressed”.
As described above, pressing the shutter button of the digital camera is a two-step operation: “start of pressing” to “half-pressing”; and “half-pressing” to “complete pressing”. By “half-pressing”, focusing is performed, and exposure conditions, such as shutter speed and aperture, are determined. Whereas, by “complete pressing”, shooting is executed.
Subtle control of force is required to operate the shutter button 2. Specifically, the shutter button 2 has to be smoothly pressed during the entire stroke from the “start of pressing” to the “complete pressing”. Accordingly, the coil spring 6p for biasing the shutter button 2 is required to apply a small and stable repulsive force during the entire stroke of the shutter button 2. Therefore, the coil spring 6p is normally formed by a spring material with a small wire diameter and a large winding diameter.
To achieve the small and stable repulsive force during the entire stroke of the shutter button 2, it is necessary to bring the coil spring 6p into uniform contact with both the guide members 4 and 5. To this end, the coil spring 6p must be aligned with the boss 24, i.e., the central axis of the coil spring 6p coincides with the central axis of the boss 24.
However, as the shutter button 2 is pressed repeatedly, the coil spring 6p might be displaced in the diameter direction of the shutter button 2 as shown in FIG. 11. When the coil spring 6p is displaced, it cannot be in uniform contact with both the guide members 4 and 5, so that the pressing force is biased and the shutter button 2 cannot be smoothly pressed.
In addition, the diameter D6 of the coil spring 6p is large as compared to the outer diameter D24 of the boss 24, and therefore the coil spring 6p is, for example, displaced or inclined in the diameter direction of the shutter button 2 during integration of the shutter button 2 into the housing 3, so that the integration is carried out with the coil spring 6p being stuck on the boss 24 or a guiding boss 25, for example. In such a case, the shutter button 2 cannot be properly pressed or returned. Therefore, assembling the switch pressing mechanism RSp and integrating it into the digital camera 1a are difficult tasks that require extreme caution.